Engine - M20 Description/Features
GROUP 11Engine
Bulletin Number
11 05 86 (2056)
Montvale, NJ
March 1987
Technical Dept.
SUBJECT:
2.5 Liter Fuel Injection Engine (M 20)
MODELS:
325i
A high performance 2.5 liter engine has been developed for the 3 - Series models. This engine has the same bore diameter (84mm) as the 2.7 liter eta engine. To reduce the displacement of the 2.7 liter block, a new crankshaft was cast with a stroke of 75mm - previously 81 mm. Because of the low weight of the engine, an extra bonus was the improved vehicle power to weight ratio. Maximum power output is 168 hp (SAE net) at 5800 rpm, maximum torque is 222 Nm at 4300 rpm.
One of the many new features is the redesigned cylinder head. The water jacket is larger and flow tested to provide good cooling under extreme temperatures and driving conditions. The a intake and exhaust ports, as well as the valves, have been enlarged for greater flow rates. All ports are machined to reduce flow resistance.
A new camshaft design gives both higher power and torque. Dual springs for each valve assure precise opening and closing of the valves even at very high engine speeds.
The combustion chamber, in combination with the new, oval shaped piston crown, was redesigned to obtain short flame paths and achieve optimum ignition and combustion conditions. Component tolerances have been reduced by performing additional machining operations.
Intake and exhaust manifolds have been changed in cross section to match the cylinder head ports and, in length and shape, to conform with the general engine concept.
An engine oil cooler is standard for this engine and the oil capacity is increased by 0.75 liters. For oil circulation the oil filter head is equipped with a thermostatic valve as well as connections for feed and return lines (arrow) to the oil cooler.
The air filter housing is made of plastic to save weight, reduce noise and avoid corrosion. The compact radiator has straight connections and is of the latest aerodynamic design. Its small external radiating surface is compensated for by the greater number of fins.
To meet air requirements, the throttle housing has a diameter of 60 mm. Air/fuel mixture and ignition timing are regulated by the new Digital Motor Electronics M1.1 with semi-sequential fuel injection. The system is basically the same as used in the new 735i (E32) - see Technical Reference Bulletin 12 O1 86 (2051) - and features an electronically regulated idle valve and fuel evaporative system, a single sensor for engine speed and reference signals, half sequential fuel injection and adaptive idle speed regulation.
NOTE
The non-adjustable M1.1 system with its enlarged adaptive, i.e. self learning abilities, compensates for changed conditions to a far larger extent than the previous Motronic systems.
IN CASE OF IDLE/IDLE MIXTURE PROBLEMS, NO ATTEMPTS SHOULD BE MADE TO CHANGE IDLE SPEED/IDLE MIXTURE BY TRYING TO MANIPULATE THE TAMPER PROOF BYPASS SCREW IN THE AIRFLOW METER OR TO ALTER THE THROTTLE BUTTERFLY PLATE POSITION.
In case of problems, verify that all components which might influence idle/idle mixture work properly according to specifications and troubleshooting procedures. If all the applicable components work properly and there are no other influences (such as unmetered air, improperly adjusted intake/exhaust valves, extremely diluted oil, etc.) and idle/idle mixture problems persist, the idle valve and/or Motronic Control Unit must be replaced.
All information contained in this publication is based on the latest information available at the time of printing. There is no updating service. Refer to the Service Information Bulletins, Technical Data, Nominal Values and Repair Manuals for the latest information.